1. Field of Invention
This invention relates to the testing of microwave radio systems, and in particular to apparatus for converting the test frequency from a 70 MHz IF frequency, normally employed in commercially available radio link test sets, to a 35 MHz IF frequency so that the commercial test set may be used to test radio equipment that operates with a 35 MHz IF.
2. Background of Invention
In the manufacturing, installation and maintenance of microwave radios, it is often necessary to test certain parameters such as amplitude response, group delay response, linearity and return loss under swept frequency conditions. These measurements are most commonly made by using one of the several commercially available test sets such as the GTE Italia CSM 221/C-222C Radio Link Test Set, the Hewlett-Packard 3710/3702 Link Analyzer, and the Siemens K1005/K1046 Sweep Frequency Test Sets.
In order to fully test a microwave radio, the aforementioned parameters must be measured from baseband to baseband, baseband to IF, IF to baseband and IF to IF. In the majority of the radios, the IF is at 70 MHz and thus most commercially available test sets are designed to operate using a 70 MHz IF with a typical bandwidth of .+-.20 MHz. There are instances, however, when a lower IF frequency such as 35 MHz is desirable, particularly in narrow band radio systems. In such cases, it would be commonly supposed that a simple mixer down converter could be employed between the test set and the input to the radio system and a mixer up converter employed between the output of the radio system and the input of the test set. However, since the 70 MHz is an integral multiple of 35 MHz, a lot of undesired mixing products will appear in the output of both the down converter and the up converter. In addition, the oscillators must be frequency stable in order to maintain a coherent, fixed, frequency relationship. Finally, unless a dual conversion scheme is used, which generates even more spurious products, an inversion in the sense of a sweep signal from the 70 MHz generator is obtained. These disadvantages are overcome by the use of the present invention.